Tunable semiconductor light sources such as lasers are in high demand for various applications such as countermeasures, remote sensing, environmental monitoring, and industrial process control. In tunable lasers, the wavelengths of the emitted radiation can be varied in a controlled manner.
Tunable mid-infrared (MID-IR) lasers in the wavelength region between 3 and 12 microns are now in high demand. Molecular absorption lines of various chemical substances such as water, carbon monoxide, sulfur, carbon and nitrogen dioxides, methane, nitric oxide, acetylene, ozone, ammonia, formaldehyde, etc. are within the specified wavelength range. The magnitude of the radiation absorption at a resonant wavelength is proportional to the concentration of the corresponding substance. The wavelength of the radiation source should be exactly matched to the absorption line of the substance detected, therefore, the tuning range of the radiation source is one of the most important parameters of the device.
There are two basic ways to control the frequency of radiation. The first method uses the change of the characteristic frequencies of light emitter material or structure as a result of changing the structure, temperature, current, voltage, strain or other control parameters. The second method involves selection of different frequencies from a sufficiently broad emission spectrum of the light emitter. The latter approach is mostly applicable to semiconductor lasers and is based on the control of the frequency dependent cavity loss and/or effective refraction index, which can be used to select the emission frequency. This method can be realized in an external cavity semiconductor laser.
The first method generally provides wider tuning range than the second, since in the latter case the tuning range is restricted by the width of the emission or amplification spectrum of the emitter. Conventional tunable semiconductor emission sources in the MID-IR range are lead-salt and antimonide-based lasers and light-emitting devices (LED), and recently implemented quantum cascade lasers (QCL). The lead salt laser diodes cover a spectral range from 3 to 30 microns. Rough wavelength tuning is done by controlling the device temperature. The tunability range of a single device can reach up to 10% of the central wavelength. Diode lasers based on (AlGaIn)(AsSb) system work in the 2-3 micron range (Simanowski S, Mermelstein C, Walther M, Herres N, Kiefer R, Rattunde M, Schmitz J, Wagner J, Weimann G, Journal of Crystal Growth, 227, 595 (2001)) with the temperature tuning wavelength range amounting to about 4 to 5% of the central wavelength.
One of the ways to tune the laser wavelength continuously is to control the effective refractive index of the laser mode. A conventional approach achieves control over the effective refractive index through the temperature of the device. A drawback of this method is a low tuning speed, which is limited by the thermal mass of the device.
In addition to temperature tuning, injection current tuning is commonly used for fine wavelength adjustment. Laser tunability over 7.5 cm−1 by changing the bias current and 3.5 cm−1 by changing the heat sink temperature have been reported for InAsSb cw lasers emitting at 3.3 μm (V. Sherstnev, A. Krier, A. Popov, P. Verle, Appl. Phys. Lett. 80, 3676 (2002)). Despite the fact that temperature control provides a good method for coarse wavelength tuning, it is electrical tuning that provides the finest frequency adjustment and the fastest time response. The range of the electrical tuning in both lead salt and antimonide based MID-IR diode lasers and LEDs is relatively small, so that the electrical tuning in these devices can be used only in combination with temperature tuning, which sometimes provides a wide enough tuning range.
The physical reason for electrical wavelength tuning can be either the change of the effective refraction index of the active area with the bias current or the change of the optical transition energy due to the change of the voltage drop across the active region (Stark shift). The Stark effect is an effective tool for broad wavelength tuning. Quantum cascade unipolar intersubband light emitting devices electrically tunable in the 8-13 micron range were reported by C. Sirtori, F. Capasso, J. Faist, D. Sivco, A. Hutchinson and A. Cho (APL, v.66, 4 (1995)).
Light-emitting diodes with a Stark tuning range over 100 nm in the 900 nm spectral region have been recently demonstrated (N. Le Thomas, N. T. Pelekanos, Z. Hatzopoulos, E. Aperathitis, R. Hamelin, Appl. Phys. Lett., 81, 1582 (2002)). The possible application of this principle to laser tuning was reported by J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson and A. Y. Cho (Nature, v.387, 777 (1994)), N. Le Thomas, N. T. Pelekanos, Z. Hatzopoulos, E. Aperathitis and R. Hamelin (Appl. Phys. Lett., 83, 1304 (2003), and Yu. Vasilyev and S. Suchalkin (Electron. Lett., 35, 1563 (1999)).
However, the suggested schemes have serious drawbacks. The design suggested by N. Le Thomas, N. T. Pelekanos, Z. Hatzopoulos, E. Aperathitis, R. Hamelin in Appl. Phys. Lett., 83, 1304 (2003) utilizes the emitter based on a “rectangular” type I quantum well, so the Stark shift is a weak, second-order effect. In the designs suggested by J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, A. L. Hutchinson and A. Y. Cho (Nature, v.387, 777 (1994)) and by Yu. Vasilyev and S. Suchalkin (Electron. Lett., 35, 1563 (1999)), the charge accumulation region is not separated from the active layers of the emitter. This makes it difficult to use such designs for laser wavelength tuning since the carrier concentration in the emitter is pinned after the laser generation onset, and hence, the generation wavelength cannot be controlled through the Stark shift unless a controlled optical loss is introduced into the laser cavity.